1. Field of the Invention
The present invention generally relates to transient suppression in electronic circuitry, and more particularly to a semiconductor device utilizing a heterojunction bipolar transistor structure having a breakdown voltage below 5 volts.
2. The Prior Arts
Protection means against transient voltages such as those arising from electrostatic discharge (ESD) are commonly employed in electronic circuitry. Zener diodes are well-known ESD protection devices, providing active voltage clamping for both positive and negative polarity in a single discrete component. It is based on a mature technology and is available across a full range of Zener breakdown voltages (Vz). However, Zener diodes with Vz less than 5V suffer from relatively high differential resistance and high capacitance per unit device area. As modern high-speed applications impose ever increasing constraints on the maximum allowed parasitic capacitance, the use of such Zener diodes in modern device circuits is therefore significantly restricted.
Punch-through diodes, essentially bipolar transistors, offer an interesting candidate to replace these Zener diodes. These transient suppression devices rely on the punch-through effect between the emitter and the collector. At certain bias between the n+emitter and the n+collector, the p-type base is fully depleted and then the electron potential barrier between the emitter and the base is reduced, causing current from the emitter to the collector to increase significantly. The punch-through effect shifts the clamping voltage to an upper value about 3V U.S. Pat. No. RE38,608 by Yu et al. teaches one punch-through diode capable of suppressing transient voltages below 5V The transient suppression device by Yu et al. has a base including a p+region and a p−region, both sandwiched between an n+emitter and an n+collector. The base has varying doping concentrations so as to improve leakage and capacitance characteristics of the transient suppression device. According to the specification of Yu et al., diffusion doping is used to form the required doping profile of the punch-through diode.
On the other hand, heterojunction bipolar transistors (HBTs), a type of bipolar transistor, are commonly used for high-frequency radio applications. The HBTs form a heterojunction between an emitter region and a base region, thereby making the energy bandgap of the emitter wider than that of the base. The feature of HBTs resides in that, since there is a substantially low injection of carriers from the base to the emitter, it is possible to increase the doping concentration in the base while maintaining the emitter injection efficiency at a high level. Therefore, it is possible to narrow the base width and lower the internal base resistance. As a result, it is possible to improve the resistance of the transient suppression device.
Conventionally, HBTs are operated well below their common-emitter breakdown voltage, and a high common-emitter breakdown voltage is commonly considered a positive performance indicator for HBTs. However, for people skilled in the art, it is well known that HBTs, due to its high base doping concentration and high injection efficiency, is already quipped with the required positive feedback mechanism for breaking down. Therefore, if the breakdown voltage of HBTs could be reduced, HBTs could be a potential candidate for transient protection.